What happens if you accidentally miss two weeks of namenda xr?

My grandfather takes namenda xr 28mg and it was not put in his pill organizer for 2 weeks straight (this was an accident). As soon as we realized that he hadn't had it in 2 weeks, we put it back in his pill organizer as usual. In those 2 weeks he seems to have gotten worse, started hallucinations and not remembering much at all. Much worse than before. He has been back on the namenda xr for 6 days now and still not improving. Is there anything we need to do to help get him back to where he was 2-3 weeks ago?